This invention relates to voice processing systems, and more particularly to echo cancellation methods and apparatus for use in voice processing systems.
Voice processing systems such as voice mail systems are typically connected to telephone transmission networks. Such networks typically have some transmission line echo, despite industry standards and efforts which are aimed at keeping this problem under control. Echo means that some or all of the transmitted signal is returned to the transmitter after some delay and generally with substantially reduced amplitude. There are many variables associated with echo, and each of these variables is subject to change from one telephone call to the next. For example, the amount of time delay involved, the frequency response, and the amplitude of the returned signal are all echo parameters which can vary considerably from one telephone call to the next, thereby making it impossible to design a single, static circuit which will effectively cancel echo under all conditions.
The typical voice processing system must both transmit signals (such as previously recorded voice messages) via a telephone line, and also automatically respond to signal information (such as dual tone multifrequency ("DTMF") tones) received via that same telephone line. For example, in response to transmission of a prerecorded message, the recipient of that message may use DTMF tones to instruct the voice processing system to perform certain functions such as replay the message, erase the message, go to the next message, etc. The ability to accurately detect and correctly respond to received DTMF tones is therefore essential to proper operation of a voice processing system. However, the echo of transmitted voice messages can sometimes resemble DTMF tones and cause prior art voice processing systems to malfunction. The echo may also be added to incoming DTMF signals, thereby corrupting those signals and preventing their detection.
In view of the foregoing, it is an object of this invention to provide improved echo cancellation methods and apparatus for use in voice processing systems.
It is a more particular object of this invention to provide echo cancellation methods and apparatus for use in voice processing systems which determine the echo characteristics of the telephone transmission line involved in each telephone call and automatically adapt the voice processing system to substantially cancel echoes having those characteristics.